Head badge and Title must be wrong
My r50
552459 was built 8/11/55
550264 was built 3/14/55
your
550375 would be april or may 1955
You can email bmw archives with 550375 to get build date
Dont have url address with me
US models not built until 67 68,69 as you stated and not sure when r50s were built but definitely not in 1955

You can click on "BMW Models" on the left panel and type in say R50 or R50US. It will take you to a spec sheet as well as a PDF file that has been uploaded listing VIN, model, build dates that I've collected over the years. This will give a rough idea of range and transition of models. It's woefully out of date and likely has errors, but that's what I had at the time. I can upload new stuff as time permits.